This invention relates to communications with a mobile user, and in particular to such communications in which the link to the mobile user is via a satellite or satellites.
US 4189675 proposes a satellite communications method and apparatus for communicating with mobile users using a network of satellite in predetermined orbits. It would be possible to provide a complete communications network using only satellites, by utilising satellite-to-satellite links. However, both power and bandwidth are at a premium in satellite communications, and the capacity of such a network would therefore be small. It has therefore been proposed to employ ground components to form part of the link between one user and another.
A problem which arises is that of knowing where a mobile user is, in other words, to which satellite (and, with multi beam satellites, which beam) a call to that user should be directed. This problem is similar to that arising in the context of international terrestrial cellular radio systems, such as the GSM system. In that system, a register is maintained in a store of the locations of mobile users.
EP 0562374 and EP 0568778 are believed to describe the call set up method used in the xe2x80x9cIridiumxe2x80x9d proposed satellite cellular mobile communication system. They describe a network in which information on the position of a mobile user is maintained at the ground switching office which is currently serving the mobile user, and also at a xe2x80x9chomexe2x80x9d station for each particular mobile subscriber. When a call is placed to a particular mobile subscriber, either the satellite switching office to which the call is first routed is the/one currently serving the subscriber (in which case the call is placed directly via the satellite to the subscriber), or the switching office contacts the xe2x80x9chomexe2x80x9d station for the subscriber, and obtains the details of the current switching office serving the subscriber, to which the call is routed (although it is not clear in what manner the routing occurs).
An alternative description of the xe2x80x9cIridiumxe2x80x9d proposals is given in the paper xe2x80x9cThe Iridium (TM) system personal communications anytime, any placexe2x80x9d J. E. Hatlelid and L. Casey, Proceedings of the Third International Mobile Satellite Conference IMC 93, 16-18 June 1993, pages 285-290, which reveals that it is proposed that the Iridium satellite cellular system is intended to operate with terrestrial cellular radio, and that calls will be transmitted via a satellite only if terrestrial cellular communication is not available.
An alternative proposed satellite cellular system is described in xe2x80x9cThe Globalstar Mobile Satellite System for Worldwide Personal Communicationsxe2x80x9d, R. A. Weideman, pages 291-296 of the Conference Proceedings mentioned above. Two alternative access network schemes, invented by the author of that paper, and believed to have been proposed for use in the proposed Globalstar system, are described in EP 0536921 and EP 0506255. In the former of these, each mobile user is allocated a xe2x80x9chomexe2x80x9d gateway station, containing information on that user. Additionally, each gateway contains information on all mobile users currently in its area.
Calls are directed to the users home gateway station. If this is the gateway station within the area of which the user is currently located, the call is then routed to the mobile user by satellite. If not, the database held at the home gateway station is consulted and the call is re-routed from the home gateway to the active gateway within the area of which the mobile user is located. Details of the rerouting are not given; it is presumably via the public network.
EP 0506255 is similar, but the user information and processing intelligence is located in the satellites, rather than in ground stations.
Both these latter two documents propose to use the satellite link only when a user has registered as xe2x80x9croamingxe2x80x9d outside his normal area of operations; whilst inside this area, he is served by a terrestrial cellular communications network.
None of the foregoing publications take account of the technical problems which may arise when a satellite cellular communications system is used for connection to a roaming mobile user who may be located at one of a number of widely dispersed points on the Earth, and who may be called from a fixed telephone at any point across the earth via a public switched telephone network (PSTN).
To give an example, suppose that in the system described in EP 0536921, a mobile user has a home gateway in Australia, but is roaming in the United Kingdom (i.e. on a diametrically opposed point on the planet), and that he is called from a user in the UK. The call would first be routed through the international PSTN from the UK to Australia, to the home gateway, whence it would be re-routed by a further link back to an active gateway in or near the UK, then finally relayed via the satellite to the mobile user. When it is borne in mind that each of the links between the UK and Australia may involve at least one satellite connection via, for example, satellites operated by INTELSAT, it will be seen that at least three xe2x80x9chopsxe2x80x9d, each comprising an earth-satellite-earth round trip are involved. The total delay in the link thus can rapidly become very substantial, with consequent rapid degradation in the perceived quality of the connection. Further, other types of noise, distortion or echo degradation of the signal may also come into play.
The present invention provides a satellite mobile communications system in which a plurality of terrestrial gateways are interconnected by a dedicated terrestrial ground network; calls to mobile users in a category of users who are entitled to roam globally are routed through the dedicated network to the ground station serving the satellite currently serving the mobile user; and calls directed to a mobile user who is in a category of users not entitled to roam outside a limited geographical area are directed to the ground station serving that geographical area via another network (e.g. the PSTN on which the call arrived).
Thus, for mobile users who may genuinely be located at dispersed locations over time, the call is routed via a dedicated, controlled network which may, for example, consist entirely of terrestrial links, so that the quality of the link to the earth station serving the satellite serving the mobile user can be controlled.
On the other hand, users who will not require global roaming (and whose position is therefore known) can be served predominantly through -the PSTN, since a relatively direct and short route through the public switched telephone network can be set up.
According to another aspect of the invention, there is provided a method of interconnecting a satellite mobile communications system user terminal and another telecommunications user terminal, comprising selecting one from a plurality of possible earth stations via which said satellite system user terminal may communicate via a satellite, and interconnecting the two user terminals via said selected earth station.
The selection of the appropriate ground station may, in one embodiment, be made based on a prediction of link quality taking into account the satellite system user position and the ephemerides of the satellites serving each earth station. It may equally be made taking into account, additionally or alternatively, the measured quality or strength of the link obtainable via each of a plurality of earth stations.
In an embodiment, where several earth stations provide an acceptable link quality, the choice may be made so as to minimise the route to be taken through the ground network (e.g. by making the selected earth station the same as the source or destination earth stations of an incoming or outgoing call, respectively) or to maximise quality through the ground network (e.g. by avoiding long distance links such as satellite links where possible).